In response to rumours that the 'black screen' effect plaguing some Windows 7 users was caused by a security update, Microsoft said that malicious software was the likely cause of the problem. 'We've conducted a comprehensive review of the November Security Updates, the Windows Malicious Software Removal Tool, and the non-security updates we released through Windows Update in November. That investigation has shown that none of these updates make any changes to the permissions in the registry. Thus, we don't believe the updates are related to the 'black screen' behaviour described in these reports.